Atmospheric gases are non-toxic, but increases in concentration - especially that of oxygen - have an effect upon life and combustion processes.

Although not flammable in itself oxygen does support combustion while nitrogen and argon inhibit combustion.

If good practice is not observed accidents may happen because changes in concentration cannot be detected by people.

When these gases are in the liquid state, it is necessary to bear in mind the extremely low temperature involved (less than -180 0C at atmospheric pressure). They can rapidly cause cold burns and make certain materials brittle, which may lead to structural failure.

Hazards from oxygen deficiency

Oxygen is essential to life, and it is vital to ensure that adequate oxygen is present in any atmosphere being breathed. A healthy person may survive a short exposure to oxygen content as low as about 16%. Oxygen deficiency gives you no relevant signals and will not be detected by people.

Unconsciousness within a few minutes Recovery possible if carried outdoors immediately

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Unconsciousness almost immediately

Causes and avoidance of oxygen deficiency

To avoid oxygen deficiency pay careful attention to the following:

Leakage of gases other than oxygen automatically leads to oxygen deficiency. Newly assembled equipment which uses inert or any other gas should be thoroughly checked for leaks using a timed gas pressure drop test, supplemented by testing with an approved leak test fluid compatible with the equipment for which it is being used.

All equipment, for instance piping and hose connections, should be properly fitted. Hoses and other equipment should be kept leak-tight and protected from damage. All maintenance and repair work should be carried out by experienced and fully skilled personnel.

When the work period is over, the cylinder valve or piped supply stop valve must be turned off in order to avoid possible leakage between two working periods. The valves on welding equipment should not be relied upon as shut-off valves for the gas supply. Gas cylinders in use should be protected against rough handling, knocks or pulling over.

A small amount of liquefied gas can give off a large amount of gas. Consequently, liquid spillage can rapidly cause oxygen deficiency in confined spaces, pits etc. Tanks and equipment for storage and handling of liquid gases should be inspected carefully and maintained in accordance with the relevant regulations and recommendations.

Vented gases often have low oxygen content, and work should not be carried out in such atmospheres.

Oxygen deficiency will arise when preparing plant items such as vessels for repair by purging with nitrogen or other inert gases.

Processes such as food cooling, ground freezing, cryogenic surgery and blood plasma preservation, in which the vaporization of liquid nitrogen is involved, lead automatically to oxygen-deficient atmospheres. People should not enter such areas, even if the atmosphere is only slightly deficient in oxygen, unless adequate breathing equipment is used.

All gas welding and heating processes take oxygen from air and can lead to deficiency unless the volume of workspaces and their ventilation is sufficient.

Removal of argon, carbon dioxide or any other cold gas from large vessels and deep pits can be difficult due to the relatively high density of the gas compared with air. Air introduced into the bottom of such spaces tends to float up through the dense gas without displacing it. This means that purging can take much longer than expected.

Detection of oxygen enrichment or deficiency

When working in rooms where the oxygen content can change to a dangerous extent during the working period, continuous measuring methods must be used.

There are measuring instruments that show increases and decreases in the oxygen concentration of the ambient atmosphere. In confined spaces the measuring instrument should be located as close to the worker as possible; it is recommended that the worker has a portable measuring instrument attached to the working clothes.

Discontinuous measuring methods may only be used if the time between any two measurements allows the tendency for dangerous changes in oxygen content to be detected quickly enough.

Other gases The safety of a space does not depend on oxygen content alone, but can be affected by other gases such as fuel gases and nitrogen oxides when using cutting or heating torches. These should be analyzed as necessary.

Preventive measures

Apparatus used for the manufacture, distribution and utilization of inert gases must be installed and identified in accordance with the recommendations of the industrial gas industry, and must comply with whatever regulations are applicable.

Any leaks must be dealt with by persons who have been adequately trained and who have the proper equipment.

Information should be provided on actions to be taken by personnel and first aiders in the event of an incident.

Operating personnel must at all times obey works rules and regulations and, where called for, protective equipment must be worn.

All people who work in spaces where oxygen deficiency or enrichment can occur should be given adequate instructions as to the risks involved, special attention being drawn to the insidious nature of the risks due to the rapidity of their effects coupled with the fact that an operator may be completely unaware of the potential danger he is exposed to.